The collapse of the Variscan belt: a Variscan lateral extrusion thin-skinned structure in NW Iberia

Silva, Ícaro Días da; Clavijo, Emílio González; Montes, Alejandro Díez

doi:10.1080/00206814.2020.1719544

Cited by 21 publications

(10 citation statements)

References 140 publications

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“…In some places of the CIZ, the lower crust was significantly thinned by pervasive extension and widespread melting processes (Alcock et al., 2015; Ayarza et al., 2021; Díez Fernández & Pereira, 2016; Escuder Viruete et al., 1998; Martínez Catalán et al., 2014), also evidenced by the intrusion of late‐Variscan granitoids (e.g., Bea et al., 2003; Orejana et al., 2012; Simancas et al., 2013). The generalized gravitational collapse of this orogen and crustal thinning was driven by broad ductile shear zones that formed under a regional N‐S to NW‐SE direction of extension (e.g., Dias da Silva et al., 2021; Doblas, 1991; Doblas, López‐Ruiz, et al., 1994; Escuder Viruete et al., 1998). Associated to these late‐Variscan low‐angle detachment tectonics and shear zones, uplift of deep and hot rocks in the footwall of these detachments caused a progressive doming of the overlying mylonitic zone resulting in an asymmetric antiform (Doblas, López‐Ruiz, et al., 1994; Escuder Viruete et al., 1994).…”

Section: Discussionmentioning

confidence: 99%

“…In the central and northern parts of the Iberian Massif, the Variscan deformation yielded three generations of contractional (C1, C2, and C3) and two extensional (E1 and E2) structures (Martínez Catalán et al, 2014). The stages C1, C2, and E1 partly overlap, with the age of each deformation phase being progressively younger from the CIZ toward the Cantabrian zone (e.g., Dallmeyer et al, 1997;Dias da Silva et al, 2021). C1 (dated in the CIZ between 360 and 350 Ma; Azor et al, 2019;Martínez Catalán et al, 2014) is characterized by a heterogenous deformation with recumbent and upright folds (e.g., Diez Balda et al, 1990) and is the response of the arrival of the Variscan tectonic front to the autochthonous terrains (Dias da Silva et al, 2021).…”

Section: Geodynamic Evolutionmentioning

confidence: 99%

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Crustal Imbrication in an Alpine Intraplate Mountain Range: A Wide‐Angle Cross‐Section Across the Spanish‐Portuguese Central System

et al. 2022

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Intraplate ranges are topographic features that can occur far from plate boundaries, the expected position of orogens as described in the plate tectonics theory. To understand the lithospheric structure of intraplate ranges, we focused on the Spanish‐Portuguese Central System (SPCS), the most outstanding topographic feature in the central Iberian Peninsula. The SPCS is an Alpine range that exhumes Precambrian‐Paleozoic rocks and is located at >200 km from the northern border of the Iberian microplate. Here, we provide a P‐wave velocity model based on wide‐angle seismic reflection/refraction data of the central SPCS (Gredos sector). Our results show: (a) a layered lithosphere characterized by three major interfaces: Conrad, Mohorovicic, and Hales discontinuities, (b) an asymmetry of the crust‐mantle boundary under the SPCS, (c) the extent of the Variscan batholith forming the main outcrops of Gredos, and (d) the thinning of the lower crust toward the south. This model suggests that the exhumation of the SPCS basement was driven by a south‐vergent thick‐skinned thrust system, developed in the southern part of the SPCS and that promoted crustal imbrication and a Mohorovicic discontinuity's offset under the SPCS. Thus, the deformation mechanisms of the crust seem to be controlled by the presence of the late‐ to post‐Variscan granitoids that assimilated the Variscan mid‐crustal detachment creating a new rheological boundary. This tectonic structure allowed the formation of Alpine crustal‐scale thrust systems that eased coupled deformation of the upper and lower crust, leading to limited underthrusting of both crustal layers.

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Section: Discussionmentioning

confidence: 99%

Section: Geodynamic Evolutionmentioning

confidence: 99%

Crustal Imbrication in an Alpine Intraplate Mountain Range: A Wide‐Angle Cross‐Section Across the Spanish‐Portuguese Central System

et al. 2022

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“…Our geochemical, isotopic and geochronological findings allowed us to (i) complement previous information about Visean and Bashkirian magmatism in the OMZ; (ii) establish the timing and the compositional changes of Carboniferous arc-related magmatism in the OMZ; and (iii) discuss the tectonic framework of OMZ Carboniferous magmatism during the process of the convergence of the Laurussian and Gondwanan continental margins; this Carboniferous arc magmatism was probably emplaced in the OMZ (upper plate) immediately after the closure of the Rheic Ocean, following the onset and development of the Paleotethys subduction. From the perspective of the temporal and spatial transition from one locally superimposed orogenic cycle to another, it may be assumed that the onset of Carboniferous OMZ arc magmatism may have overlapped chronologically with (i) the magmatic activity recognized in the South Portuguese Zone (i.e., Pyrite belt) that took place after the Laurussia-Gondwana collision (i.e., Variscan orogeny) as a result of the slab break-off of the Rheic oceanic lithosphere beneath the Laurussian margin during the Tournaisian [23]; and (ii) the production of magmatism in the Central Iberian and West Asturian-Leonese zones that mainly derived from the extensional collapse of the Variscan orogen in the Serpukhovian-Bashkirian [53][54][55], and the subsequent development of the Iberian orocline from Moscovian to Gzhelian times [56,57].…”

Section: Discussion: Sources Of the Carboniferous Magmatic Arcmentioning

confidence: 99%

Changing Carboniferous Arc Magmatism in the Ossa-Morena Zone (Southwest Iberia): Implications for the Variscan Belt

et al. 2022

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Carboniferous magmatism in southwestern Iberia was continuously active for more than 60 m.y. during the development of the Appalachian-Variscan belt of North America, North Africa and Western-Central Europe. This collisional orogen that records the closure of the Rheic Ocean is essential to understanding the late Paleozoic amalgamation of the Pangea supercontinent. However, the oblique convergence between Laurussia and Gondwana that lasted from the Devonian to the Carboniferous was likely more complex. Recently, a new tectonic model has regarded the Iberia Variscan belt as the site of coeval collisional and accretionary orogenic processes. Early Carboniferous plutonic rocks of southwest Iberia indicate arc magmatism in Gondwana. The Ossa-Morena Zone (OMZ) acted as the upper plate in relation to the geometry of the Paleotethys subduction. This active accretionary-extensional margin was progressively involved in a collisional phase during the Late Carboniferous. Together, the Évora Massif and the Beja Igneous Complex include three successive stages of bimodal magmatism, with a chemical composition indicative of a long-lived subduction process lasting from the Tournaisian to the Moscovian in the OMZ. The earliest stage of arc magmatism includes the Tournaisian Beja and Torrão gabbro-dioritic rocks of the Layered Gabbroic Sequence. We present new geochemical and Nd isotopic and U-Pb geochronological data for magmatic rocks from the Main (Visean-Serpukhovian) and Latest (Bashkirian-Moscovian) stages of arc magmatism. Visean Toca da Moura trachyandesite and rhyolites and Bashkirian Baleizão porphyries and Alcáçovas quartz diorite share enriched, continental-crust like characteristics, as indicated by major and trace elements, mainly suggesting the addition of calc-alkaline magma extracted from various mantle sources in a subduction-related setting (i.e., Paleotethys subduction). New U-Pb zircon geochronology data have allowed us to establish a crystallization age of 317 ± 3 Ma (Bashkirian) for Alcáçovas quartz diorite that confirms a temporal link with Baleizão porphyry. Positive εNd(t) values for the Carboniferous igneous rocks of the Beja Igneous Complex and the Évora gneiss dome indicate production of new juvenile crust, whereas negative εNd(t) values also suggest different grades of magma evolution involving crustal contamination. The production and evolution of Carboniferous continental crust in the OMZ was most likely associated with the development of an active continental margin during the convergence of the Paleotethys Ocean with Gondwana, involving juvenile materials and different grades of crustal contamination.

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“…In the studied area, the three structural units considered in this work (UPa, LPa and CIZ) underwent a regional Barrovian metamorphism (M1) through the early Variscan compressive events (C 1 +C 2 on the Alcock et al, 2015 proposal) which were later followed by a complex extensional (E 1 -M 2 ) and compressive (C 3 -M 3 ) tectonothermal history (Azor et al, 2019;Dias da Silva et al, 2020). Some specialized studies were performed to discriminate if the metamorphic grade at the syn-orogenic units was lower than in the pre-orogenic units by illite crystallinity (Antona and Martínez Catalán, 1990) and Colour Alteration Index in conodonts (Sarmiento and García-López, 1996;Sarmiento et al, 1997) but no conclusive results were attained.…”

Section: Martínezmentioning

confidence: 99%

A tectonic carpet of Variscan flysch at the base of an unrooted accretion prism in NW Iberia: U-Pb zircon age constrains from sediments and volcanic olistoliths

Clavijo

Silva

Catalán

et al. 2020

Preprint

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Abstract. The allochthonous complexes of Galicia – Trás-os-Montes Zone (NW Iberia) are part of the tectonic stack that unrooted the Variscan accretionary prism. They are formed by individual tectonic slices marked by specific tectono-metamorphic evolution, which was piled up in a piggy-back thrust complex onto its relative autochthon, the Central Iberian Zone (CIZ). Consequently, allochthony decreases towards lower, more external and younger thrust sheets. The lowermost unit of this pile of slivers is known as Schistose Domain or Parautochthon and bears low metamorphic grade, contrasting with the higher temperatures and pressures estimated for the upper allochthonous units, but sharing the stratigraphic sequence with the underlying autochthon. The Parautochthon is divided in two structural and stratigraphic sub-units: (i) the Lower (LPa) made of synorogenic flysch-type sediments with varied turbiditic units and olistostrome bodies, showing Upper Devonian-lower Carboniferous age on base of the youngest zircon populations and fossiliferous content; (ii) the Upper (UPa), composed of highly deformed pre-orogenic upper Cambrian-Silurian volcano-sedimentary sequence comparable with both the nearby autochthon and the HP-LT Lower Allochthon, laying structurally above. The UPa thrusted onto the LPa by the the Main-Trás-os-Montes Thrust; and the LPa detached from the CIZ relative autochthon by a regional structure (Basal Lower Parautochthon Detachment) which follows the favourable Silurian carbonaceous beds. A review on the detrital zircon studies of the synorogenic LPa complemented by 17 new samples geochronology is here presented. The results support the extension of the LPa underneath the NW Iberia allochthonous complexes, from Cabo Ortegal, to Bragança and Morais Massifs. Its current exposure follows the lowermost tectonic boundary between the Galicia – Trás-os-Montes (allochthon) and Central Iberian (autochthon) Zones. Youngest zircon age populations point to a maximum sedimentation age for the LPa formations ranging from Famennian to Serpukhovian and endorse the piggy-back evolution inside this unit, mimicking the general structure of the Galicia – Trás-os-Montes Zone. The zircon populations in the LPa allow constraining the sedimentary provenance areas, showing the intervention of nearby sources (mostly the UPa) and/or multiply recycled/long transport sediments with typically N-Central Gondwana age fingerprint, also found in the Lower Allochthon, UPa and Autochthon. Complementary geochronology of volcanic olistoliths trapped in the LPa sediments and of upper Cambrian to Upper Ordovician rhyolites from the UPa is also presented, showing a direct relation between the major block's source area (UPa) and the setting place (LPa). Old zircon age patterns show that the LPa sedimentary rocks were recycled from detrital rocks of the allochthon (advancing wedge) and the nearby autochthon (peripheral bulge).

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The collapse of the Variscan belt: a Variscan lateral extrusion thin-skinned structure in NW Iberia

Cited by 21 publications

References 140 publications

Crustal Imbrication in an Alpine Intraplate Mountain Range: A Wide‐Angle Cross‐Section Across the Spanish‐Portuguese Central System

Crustal Imbrication in an Alpine Intraplate Mountain Range: A Wide‐Angle Cross‐Section Across the Spanish‐Portuguese Central System

Changing Carboniferous Arc Magmatism in the Ossa-Morena Zone (Southwest Iberia): Implications for the Variscan Belt

A tectonic carpet of Variscan flysch at the base of an unrooted accretion prism in NW Iberia: U-Pb zircon age constrains from sediments and volcanic olistoliths

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